A major aim of our studies is to understand in chemical terms the differences between neurons that underly their physiological function. Our initial efforts were concentrated on the identification of synaptic transmitter compounds and learning something at the reasons for their accumulation within neurons. Our approach is to bring together physiological and anatomical techniques with micro-biochemical studies in a multi-discipline study. A simple nervous system (the lobster nervous system) was selected for these studies because of its favorable anatomy. Neurons are large, relatively few in number (compared to vertebrates) and axons and cell bodies of single physiologically identified nerve cells can be isolated repeatedly from a series of animals. This allows one to carry out detailed chemical and physiological analyses on single types of functionally identified neurons. Our past studies were concerned with gamma-aminobutyric acid, the inhibitory transmitter compound at lobster neuromuscular junctions, glutamic acid, the leading candidate for the excitatory transmitter at the same junctions, and acetylcholine, the probable lobster sensory transmitter compound. In this application we concentrate on 5-hydroxytryptamine (serotonin) and on octopamine, the phenolamine analogue of norepinephrine. These amines are associated with a lobster neurosecretory system. It may be possible to analyze this system in detail from the level of its activation to its effects on lobster behavior. One action of the amines is to modulate synaptic activity at neuromuscular junctions. Our recent studies are focussed on this modulation in an attempt to understand the underlying mechanisms.